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(CNN)Seven US military service members were wounded Saturday in an insider attack at Camp Shaheen in northern Afghanistan, the NATO-led Resolute Support Mission said.

The wounded troops have been evacuated for treatment, the mission said in a statement. No US troops were killed.

The attack appears to have been a "green-on-blue" act by an Afghan soldier, a coalition official told CNN.

Initially, an Afghan military spokesman said four foreign soldiers were wounded when an Afghan soldier opened fire on foreign troops in Mazar-e Sharif. That spokesman did not identify the nationality of the injured soldiers.

The NATO-led mission said it has opened an investigation.

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Afghanistan's Ministry of Defense also is investigating, said Abdul Qhahar Aram, head of strategic communication and public affairs for the 209 Shahin Corps.

A massive attack in April on the same Shaheen base by Taliban militants left scores of Afghan soldiers dead.

Earlier this week,a convoy carrying US and Afghan forces was struck by a roadside bomb and came under small arms fire. The convoy returned fire in self-defense, and there were no US casualties, NATO said in a statement.

Last Saturday, three US soldiers were killed and another was wounded during a joint US-Afghan military operation in Afghanistan's Achin district. The Taliban claimed responsibility for that attack.

CNN's Barbara Starr in Washington and CNN's Ehsan Popalzai in Kabul, Afghanistan, contributed to this report.